1. Field of the Invention
This invention relates to an image forming apparatus for processing supplied image data and forming an image and obtaining a final image on a recording medium. More particularly, the present invention relates to a half tone processing technique adaptable to an image forming apparatus of the foregoing type.
2. Background
Among the image forming apparatuses of the foregoing type, laser beam printers (LBP) adapted to an electrophotographic process have been used widely because of considerably high output speed and excellent quality of a formed image.
At present, monochrome laser beam printers are mainly used in the market, the monochrome laser beam printer being arranged to subject data of, for example, characters and line drawings to a binary coding process. Also a binary dither method is frequently used to process image data which must have sufficient gradation.
There recently arises a requirement for a color LBP. Furthermore, an image forming apparatus having more excellent gradation expressing performance has been required. To satisfy the foregoing requirements, a variety of image forming apparatuses have been suggested.
A first example of the conventional image forming apparatus disclosed in Unexamined Japanese Patent Publication No. Hei. 5-176163 will now be described. The first example has a structure such that blocks each of which is composed of a plurality of pixels on image data are provided; dots are enlarged in a sequential order to begin with the dot corresponding to the minimum recording pixel position given the highest priority in accordance with a predetermined spatial priority order in the block; and in a region in which dots having a specific priority are continuously enlarged, enlargement of dots having a priority lower than the specific priority is started.
The first conventional example, however, sometimes suffers from so-called jumping of density with which the areas of pixels, which are formed on a recording medium, are undesirably enlarged when the distances among pixels are too short because their potential gradients on the photosensitive member affect each other.
A second example of the conventional image forming apparatuses disclosed in Unexamined Japanese Patent Publication No. Hei. 5-284343 will now be described. The printing apparatus according to the second example has a screen angle and structured in such a way that matrices composed of threshold values are used to enlarge dots having the higher priority over dots in the sub-scanning direction. That is, the number of dots on the same scanning line is reduced so that prevention of nonuniformity is expected because a smaller number of dots are affected even if the scanning lines are scanned at positions deviated from required positions.
Although the structure of the second conventional example is effective for the apparatuses of a type having a screen angle, the structure cannot be applied to apparatuses of a type comprising a multiline screen having a screen angle of zero degrees which are commonly employed by color printers. Since the number of dots on which deviation of the scanning lines affects is simply reduced, linear nonuniformity in the apparatuses of a type having a screen angle cannot perfectly be prevented.
A third example of the conventional image forming apparatuses disclosed in Unexamined Japanese Patent Publication No. Hei. 1-138946 will now be described. The image forming apparatus according to the third conventional example is structured to prevent linear nonuniformity caused from change in the scanning speed of exposing light beams. To achieve this, the diameter of the exposing light beam spot in the sub-scanning direction is determined to be 0.6 time to 1.2 times the pitches of pixels in the sub-scanning direction. Moreover, the diameter of the exposing light beam spot in the main scanning direction is determined to be one time to two times the pitches of pixels. Thus, the printing patterns are continued in the main scanning direction.
Although the third conventional example is able to prevent linear nonuniformity caused from change in the speed of scanning using the exposing light beam in the main scanning direction, the effect of eliminating an influence of the deviation of the scanning lines as is obtainable from the second conventional example cannot be expected. On the contrary, the setting of the diameter of the exposing light beam spot and the pitches of pixels made in such a manner that the distributed quantities of light interfere with each other in the sub-scanning direction results in the overall width of the scanning lines being affected by deviation of the scanning lines. As a result, excessive nonuniformity has taken place.
The above-mentioned nonuniformity is a critical problem for the color LBP. In particular, prevention of the above-mentioned nonuniformity in the half tone density region including the skin color which is considered to be important to form a natural image is an important fact.
Nevertheless, an image forming apparatus which is capable of forming an image having an improved quality by effectively preventing linear nonuniformity and by obtaining a smooth half tone image has not been obtained.